1. Field of the Invention
The invention relates generally to a method and apparatus for acoustical imaging of cased wells.
2. Background Art
In a well completion, a casing or pipe is set in a wellbore, and a fill-material, typically cement, is forced into an annulus between the casing and a formation. The primary purpose of such cement is to separate oil- and gas-producing layers from each other, and from water-bearing strata.
FIG. 1 shows a schematic diagram of a cased well. The cased well generally includes a number of interfaces 121, 122, 123 at junctures of differing materials within a wellbore 11. A “first interface” 121 exists at the juncture of a borehole fluid 13 in a casing 14 and the casing 14. The casing 14 is typically made of steel. A “second interface” 122 is formed between the casing 14 and an annulus 15 behind the casing 14. If cement 112 is properly placed in the annulus 15, the “second interface” 122 exists between the casing 14 and the cement 112. A “third interface” 123 exists between the annulus 15 and a formation 16. The formation 16 may comprise a plurality of layers, e.g., an oil-producing layer 17, a gas-producing layer 18 and a water-bearing layer 19.
A micro-annulus 111 may appear at the second interface 122, between the casing 14 and the cement 112. A forming of the micro-annulus 111 is due to a variation of pressure inside the casing 14. Even if the micro-annulus 111 is present, the layers 17, 18, 19 may be properly sealed off by the cement 112.
However, if a void 113 appears between the casing and the formation, the cement may fail to provide isolation of one layer 17, 18, 19 from another. Fluids, e.g., oil, gas or water, under pressure may migrate from one layer 17, 18, 19 to another through the void 113, and create a hazardous condition or reduce production efficiency. In particular, migration of water into the oil-producing layer 17 may, in some circumstances, render a well non-exploitable. Also, migration of oil into the water-bearing layer 19 is environmentally and economically undesirable. Thus, imaging the annulus content, and, in particular, detecting the third interface 123 between the annulus 15 and the formation 16, may be important for reliable determination of the hydraulic isolation of the different layers of a formation.
Another need for through-the-casing imaging exists in the process of hydraulic fracturing, which typically takes place after a well has been cased, and is used to stimulate the well for production. Often, the fracturing process is accompanied by sanding, whereby certain strata of the formation release fine sand that flows through casing perforations into the well, and then up to the surface, where it can damage production equipment. This problem can be remedied if the sand-producing zones are detected as could be done, for example, with an imaging technology capable of operating through the casing.
Various cement evaluating techniques using acoustic energy have been used in prior art to investigate a description of a zone behind a thick casing wall with a tool located inside the casing 14.
First Cement Evaluation Technique from Prior Art
FIG. 2 contains a schematic diagram of a first cement evaluation technique involving acoustic waves having an extensional mode inside a casing 24. The first cement evaluation technique is described in U.S. Pat. No. 3,401,773, to Synott, et al. A logging tool 21 comprising a conventional, longitudinally spaced sonic transducer for transmitting 22 and transducer for receiving 23 is used to investigate a well 28. Both transducers operate in the frequency range between roughly 20 kHz and 50 kHz. A fill-material 25 isolates the casing 24 from a formation 26. The logging tool 21 is suspended inside the casing 24 with a cable 27.
The sonic transducer for transmitting 22 insonifies the casing 24 with an acoustic wave 27 that propagates along the casing 24 as an extensional mode whose characteristics are determined primarily by the cylindrical geometry of the casing and its elastic wave properties. A refracted wave 29 is received by the transducer for receiving 23 and transformed into a received signal
The received signal is processed to extract a portion of the signal affected by the presence or absence of cement 25 behind the casing 24. The extracted portion is then analyzed to provide a measurement of its energy, as an indication of the presence or absence of cement outside the casing 24. If a solid, e.g., cement, is in contact with the casing 24, the amplitude of the acoustic wave 211 propagating as an extensional mode along the casing 24 is partially diminished; consequently, the energy of the extracted portion of the received signal is relatively small. On the contrary, if a liquid, e.g., mud, is in contact with the casing 24, the amplitude of the acoustic wave 211 propagating as an extensional mode along the casing 24 is much less diminished; consequently, the energy of the extracted portion of the received signal is relatively high. A state, e.g., liquid or solid, of the matter behind the casing 24 is thus evaluated from the value of the energy received. This technique provides useful information about the presence or absence of the cement next to the second interface 210 between the casing 24 and the annulus.
However, the first cement evaluation technique uses low frequency sonic waves (20 to 50 kHz). Those acquainted with acoustic theory in general and sonic borehole in particular will recognize that this casing extensional mode involves vibrations of the entire cylindrical structure of the casing 24. As a consequence, there is no azimuthal resolution. The results may be plotted in a curve as a function of depth only.
Second Cement Evaluation Technique from Prior Art
FIG. 3 contains a schematic diagram of a second cement evaluation technique for investigating the quality of a cement bond between a casing 32 and an annulus 38 in a borehole 39 formed in a formation 310. The second cement evaluation technique is described in U.S. Pat. No. 2,538,114 to Mason and U.S. Pat. No. 4,255,798 to Havira. The measurement is based on an ultrasonic pulse echo technique, whereby a single transducer 31 mounted on a logging tool 37, insonifies the casing 32 at near-normal incidence, and receives reflected echoes 33.
The transducer 31 insonifies the casing 32 with an acoustic wave 34 having a frequency selected to stimulate a selected radial segment of the casing 32 into a thickness resonance. A portion of the acoustic wave is transferred into the casing and reverberates between a first interface 311 and a second interface 35. The first interface 311 exists at the juncture of a borehole fluid in a casing 32 and the casing 32. The second interface 35 is formed between the casing 32 and the annulus 38 behind the casing 32. A further portion of the acoustic wave is lost in the annulus 38 at each reflection at the second interface 35, resulting in a loss of energy for the acoustic wave. The acoustic wave losses more or less energy depending on the state of the matter 312 behind the casing 32.
Reflections at the first interface 311 and second interface 35, give rise to a reflected wave 33 that is transmitted to the transducer 31. A received signal corresponding to the reflected wave 33 has a decaying amplitude with time. This signal is processed to extract a measurement of the amplitude decay rate. From the amplitude decay rate, a value of the acoustic impedance of the matter behind the casing 32 is calculated. The value of the impedance of water is near 1,5 MRayl, whereas the value of impedance of cement is typically higher (for example this impedance is near 8 MRayl for a class G cement). If the calculated impedance is below a predefined threshold, it is considered that the matter is water or mud. And if the calculated impedance is above the predefined threshold, it is considered that the matter is cement, and that the quality of the bond between cement and casing is satisfactory.
The second cement evaluation technique uses ultrasonic waves (200 to 600 kHz). Those acquainted with acoustic theory in general will recognize that the excited casing thickness mode involves vibrations of the segment of the casing confined to an azimuthal range. The second cement evaluation technique thus provides spatial resolution as opposed to the first cement evaluation technique.
The values of the impedance may be plotted in a map as a function of a depth and an azimuthal angle. The depth and the azimuthal angle may be plotted respectively on a first and a second axis. The value of the impedance may be represented by a color.
However, the first cement evaluation technique and the second cement evaluation technique provide information predominantly on the state of the matter located at the second interface 35 only.
U.S. Pat. No. 5,763,773 to Birchak et al. discloses a multi-part logging apparatus consisting of pulse-echo and pitch-catch transducers to probe the cement outside of a casing. A pitch-catch system refers to the use of separate transmitting and receiving transducers whose alignment angle with respect to the normal to the casing is different from zero (i.e., non-normal incidence). The disclosure teaches one to align all pitch-catch transducers at angles, with respect to the normal of the casing inner wall, that are less than a shear wave critical angle of a first interface between the casing and a fluid, e.g., oil or gas, therein. Additionally, a method to evaluate the quality of a cement seal is disclosed. This method relies on quantifying the attenuation of the propagating energy between transducers.
Third Cement Evaluation Technique from Prior Art
A third cement evaluation technique is described in U.S. Pat. No. 6,483,777 to Zeroug. FIG. 4 provides an illustration of the third cement evaluation technique. A logging tool 41 comprising an acoustic transducer for transmitting 42 and an acoustic transducer for receiving 43 mounted therein is used to investigate a well 411. The transducer for transmitting 42 and the transducer for receiving 43 are aligned at an angle θ. The angle θ is measured with respect to the normal to the local interior wall of the casing N. The angle θ is larger than a shear wave critical angle of a first interface 46 between a casing 44 and a fluid 47, e.g., oil or gas, therein. Hence, the transducer for transmitting 42 excites a flexural wave A in the casing 44 by insonifying the casing 44 with an excitation aligned at the angle θ greater than the shear wave critical angle of the first interface 46.
The flexural wave A propagates inside the casing 44 and sheds energy to the fluid 47 inside the casing 44 and to the fill-material 45 behind the casing 44. A portion B of the flexural wave propagates within an annulus 410 and may be refracted backward at a third interface 412. An echo 49 is recorded by the transducer for receiving 43. A measurement of a propagation time may be extracted from a signal at an output of the transducer for receiving 43 corresponding to the echo 49.
A velocity of the wave within the annulus 410 may be calculated from the propagation time, provided that the thickness of the annulus 410 is known. The velocity of the wave depends on a nature of the acoustic wave within the annulus, which depends itself on the quality of the fill-material.
If an additional transducer for receiving (not represented on the figure) is provided at a location on the logging tool above the acoustic transducer for receiving 43, an additional signal may be produced at an output of the additional transducer for receiving. A flexural wave attenuation, may be extracted from the signal and from an additional signal. The flexural wave attenuation depends on the quality of the fill-material within the annulus 410.
The quality of the cement behind the casing 44 may be evaluated from the velocity of the wave within the annulus 410 and/or the flexural wave attenuation. The quality, e.g., a state of the matter, may be plotted in a map as a function of depth and azimuthal angle.
Since the portion B of the flexural wave propagates within the annulus 410, the corresponding signal provides information about the entire matter within the annulus 410, i.e., over an entire distance separating the casing 44 and the third interface 42.